104 



MUSCULAR TISSUE 



Any interpretation of the intercalated disks as actual intercellular struc- 

 tures (cement substance) would seem inconsistent with the myogenic theory 

 of the heart beat, which now seems largely to prevail. The present status 

 of the matter seems to be that the origin of stimulus to heart beat in 

 vertebrates is myogenic, in invertebrates probably neurogenic. The differ- 

 ence may inhere in the absence in the hearts of invertebrates of a muscular 

 coordinating structure analogous to the atrioventricular bundle of verte- 

 brate hearts. 



FIG. 114. SEMIDIAGRAMMATIC ILLUSTRATIONS OF VARIOUS TYPES OF INTERCALATED 



DISKS. 



a, from guinea pig's heart; b, from chipmunk's heart; c, from monkey's heart; 

 d, from monkey's heart; e, from guinea pig's heart;/, from chipmunk's heart. X 1200. 



Voluntary Striped or Skeletal Muscle. The unit of structure of 

 skeletal muscle is essentially the same as that described for cardiac mus- 

 cle, namely, a myofibril or sarcostyle. The fiber is likewise divisible into 

 successive sarcomeres or iiiokommata. A difference in detail inheres in 

 a greater definiteness of striation, and a greater complexity, due to the 

 presence generally of an additional disk in the J stripe. This stripe 

 or accessory disli of Engelmann (X line; nebenscheibe) bisects the 



